Ball ramp caliper brakes are a useful and convenient means of providing a braking force. Generally such brakes include a rotatable actuator and a stationary surface, each of which have depressions which are circumferentially spaced around an axis, and which become progressively deeper in one circumferential direction. Steel balls are held within these depressions and when the rotatable actuator is caused to rotate, the balls roll gradually along the depressions away from the deeper end. This in turn causes the rotatable actuator to move axially away from the stationary surface thereby applying a braking force to a disc assembly. These brakes are relatively simple in design because they do not require complex hydraulic mechanisms. Thus, the brakes are relatively dependable and easy to service and operate. Nevertheless, some deficiencies exist in prior art ball ramp brakes.
One such deficiency is the tendency for the ball ramp brakes to overheat. The disc assembly of the ball ramp brake contains a plurality of stationary discs and a plurality of rotating discs all coaxially mounted on a rotating shaft. During normal operation, the discs are separated by a clearance allowing the rotating discs to rotate freely between the stationary discs. When the brake is actuated, a vehicles momentum is converted into heat by the friction between the rotatable and stationary discs within the disc assembly. This heat is transferred to a fluid within the brake chamber, which is typically an oil. Due to the high amount of heat produced, and the lack of adequate cooling means for the fluid within the housing of the brake, there has been a tendency for the brake to overheat and, consequently, lose braking power. A previously attempted solution to the overheating problem was to provide a higher volume of lubricant within the disc chamber in order to increase the lubricant's ability to absorb heat. However, the additional volume of lubricant lowered the efficiency of the ball ramp brake.
Another deficiency in prior art ball ramp brakes relates to the braking force applied to the disc assembly by the rotatable actuator. The ramp angle of the depressions within the rotatable actuator and stationary surface are directly related to both the response time of the brake as well as the mechanical advantage generated by the actuator. In prior art ball ramp brakes, a steep ramp angle is typically provided in order to decrease the amount of rotation needed in order to engage the disc assembly. This steep angle, however, reduces the mechanical advantage of the actuator and lowers the force applied to the disc assembly. If the ramp angle is reduced in order to increase the mechanical advantage, then the shallow ramp angle increases the amount of rotation needed in order to engage the disc assembly, and thereby increases the response time and volumetric displacement required of the brake.
Yet another deficiency in prior art ball ramp brakes is the inability to use two different lubricating fluids within the housing of the brake without cross-contaminating both fluids. This inability is significant because various components of the brake assembly may perform more efficiently in the presence of different fluids. For instance, it is believed to be beneficial to use DOT-3 or DOT-4 brake fluid in a slave cylinder of the brake, while it is preferable to use a petroleum-based oil within the disc assembly. Prior art ball ramp brakes do not provide an effective means of using both fluids simultaneously by preventing mixing of the two fluids.
In view of these problems, it is evident that the need exists for a ball ramp brake providing improved cooling capabilities, an improved actuation system, and a method of separating fluid filled chambers so as to allow multiple fluids to be used.